There has widely been used a system that drives and controls an electrical apparatus serving as a load, while a power storage device, a typical example of which is a secondary battery (hereinafter also simply referred to as a battery), is being charged/discharged. For example, such a system is applied to a hybrid vehicle configured to include an engine, a motor generator that functions as an electric motor or an electric power generator, and a battery that receives electric power from and transmits electric power to the motor generator for charge/discharge.
Japanese Patent Laying-Open No. 2002-58113 (hereinafter referred to as Patent Document 1) discloses a configuration of a mechanical power output device configured such that, to allow a battery mounted on a hybrid vehicle, as described above, to fully exhibit its performance, a battery output limit is temporarily relaxed up to an instantaneous output larger than a rated output exclusively for prescribed power allowable time, when an output requested from the battery exceeds the rated output.
Similarly, Japanese Patent Laying-Open No. 2003-92804 (hereinafter referred to as Patent Document 2) discloses, as a control device for a hybrid vehicle, a configuration that variably sets permissible capacitance to be used and permissible time to be used during charge/discharge of a battery, in accordance with a running condition of the vehicle, so as to provide satisfactory driving performance and ensure durability of the battery by performing charge/discharge control for the battery in accordance with a driving request. It is particularly disclosed that, on a condition that large electric power must be supplied or recovered during short time, a margin of the battery is made smaller to satisfy a request for a charge/discharge current, and a duration of charge/discharge is set shorter to prevent the total quantity of charge/discharge from being excessively large.
Further, Japanese Patent Laying-Open No. 11-187577 (hereinafter referred to as Patent Document 3) discloses that a limit value for charge/discharge electric power is set in accordance with a battery temperature and a power storage quantity (SOC: State of Charge), so as to perform charge/discharge with appropriate electric power suitable for a state of the battery and an environment where the battery is used.
Further, Japanese Patent Laying-Open No. 2004-215459 (hereinafter referred to as Patent Document 4) discloses a power supply control device for reducing charge time required for a plurality of connected power storage means and reducing capacitance relative to a discharge current value, and for performing charge/discharge control so as to avoid occurrence of abnormalities in the power storage means. It is particularly disclosed that, in this power supply control device, internal impedance (internal resistance) of the plurality of power storage means (batteries) connected in parallel is successively measured based on data of detected current and voltage, and the secondary battery is charged/discharged based on the determined impedance, and a maximum voltage value and a minimum voltage value.
As shown in Patent Document 4 as well, charge/discharge control for the battery must be performed so as not to allow a battery output voltage to fall outside a controlled voltage range from a minimum permissible voltage (lower limit voltage) to a maximum permissible voltage (upper limit voltage) owing to overdischarge or overcharge. Particularly in the control configuration that temporarily relaxes charge/discharge limitation in accordance with a request from a load, as in Patent Documents 1 and 2, consideration must be given at this time so as not to allow the battery output voltage to fall outside the controlled voltage range described above.
In this regard, Patent Document 1, for example, discloses that an instantaneous output permissible for short time is determined from a battery temperature and remaining capacity (SOC) at that time. Further, Patent Document 2 discloses that a current quantity relaxed under charge/discharge limitation is determined based on an SOC at present and a battery margin set in accordance with a driving circumstance.
However, in the configurations disclosed in Patent Documents 1 and 2, a battery voltage at each time point, which momentarily varies, is not considered, and hence it is difficult to accurately set a relaxed level of charge/discharge limitation permitted exclusively for short time, within a range from a lower limit voltage to an upper limit voltage. Therefore, depending on a charge/discharge history by that point in time, the battery voltage may be excessively lowered, and hence may become lower than a lower limit voltage when discharge limitation is relaxed. Alternatively, in the case where SOC estimation accuracy is degraded, the battery voltage may become lower than a lower limit voltage when discharge limitation is relaxed. In view of this, if a relaxed level of charge/discharge limitation is determined to ensure safety so as not to allow the battery voltage to be higher than an upper limit voltage and lower than a lower limit voltage, temporarily permissible charge/discharge electric power becomes small, and hence there arises a problem of not being able to obtain the best performance from the battery.